A major emphasis of mobile computing system (MCS) development is to increase battery life without increasing the battery size. This can be done by decreasing the energy consumption of the MCS. The optical storage drive of a MCS is high on the list of energy consuming components (the display screen and the hard drive, being perhaps the only worse offenders). A compact disc (CD) drive or a digital versatile disc (DVD) drive may consume up to 20% of a system's energy.
There are many optical storage disk formats including CD audio, CD-R, CD-RW, DVD-RAM, DVD-ROM, and DVD-video. These disks can be read at different rates. Optical storage drives having various drive speeds have been developed with the drive speeds usually designated in reference to a base multiplier X. The base multiplier represents the time it took to read data from a CD in its original format, which was 150 kilobytes per second (kbps). Therefore a 1X drive speed accommodated a data transfer rate of 150 kbps, a 2X drive speed accommodated a data transfer rate of 300 kbps, etc. A 1X drive speed corresponds to the disc spinning within the drive at between 200 and 500 RPMs. CDs with successively faster data transfer rates have been developed. Optical storage drives to accommodate these data transfer rates may have drive speeds of 12X, 24X or 52X. These drive speeds may correspond to proportionately higher disk rotational speeds. For example an 8X drive speed may spin the disk within the drive at between 1600 and 4000 RPMs. Some storage media such as DVD may be much denser than a CD, and therefore, be able to transfer data at much higher rates without a proportional increase in disk rotational speed.
Many optical storage drives are designed to support reading from various types of optical storage media. For example, MultiRead, MultiRead2, and Multiplay are Optical Storage Technology Association (OSTA) designations for devices that can read multiple formats such as DVD-ROM, DVD-Video, CD, CD-R, CD-RW, and others. These devices are typically fixed speed, with that speed corresponding to the highest speed that may be required for applications that the device supports. These devices may therefore spin disks much faster than required for a particular content of a disk. For example, a Multiplay device that is reading an audio CD may spin the disk at the same speed as if it were reading DVD-video. Therefore energy is being consumed to keep the audio CD spinning at a rate that may be many times faster than required to transfer audio CD data. The disk is rotating at a high rate, but only being accessed periodically as required to read the audio data.
An optical storage drive typically consumes a relatively large amount of energy to get the disk rotating at a given speed, after which the energy consumption is dependent to a large extent on the speed at which the disk is rotating.
FIG. 1 illustrates energy consumption for reading DVD-Video at, for example 12X, as compared to reading an audio CD at, for example 1X. Graph 105 shows the energy consumption for a DVD. At t0 the disk is at rest, as illustrated there is some energy consumption associated with holding the disk at rest. At t1 there is a surge in energy consumption to get the disk rotating at the desired speed. This surge is known as in-rush current and may spike as high as 3.5 watts. After the disk is rotating, at t2, there is a steady consumption of energy that may be approximately 0.75 watts. At t3 the disk returns to rest. Graph 110 shows the energy consumption for a CD audio. As shown, graph 110 has similar characteristics to that of 105, but uses less energy. At t1 the surge in energy consumption may be less than 3.5 watts. In general it may take almost as much energy to get the disk spinning, but the energy required to keep the disk rotating at a slower speed may be significantly less. For example, a speed of 1X may require only approximately 0.25 watts to maintain.
More energy is required to get the disk rotating at a higher speed and to keep it rotating at a higher speed. Therefore, Multiplay devices may consume more energy than necessary by rotating a disk at a higher speed than necessary.